Direct coupled high fidelity amplifier



Jan. 8, 1957 J. F. DOBOSY 2,777,020

DIRECT COUPLED HIGH FIDELITY AMPLIFIER Filed June 22. 1951 OUT IN V ENTOR. JOSEPH F 00305 Y 7 BY H/S A TTOFNEYS iE/CHEY& WATTS nited tates 3mmDIRECT COUPLED HIGH rmnrrrv AMPLIFIER Joseph F. Dobosy, Avon Lake, hioApplication June 22, 1951, Serial No. 232,887

5 Claims. (Cl. 179-171) My invention relates to electronic amplificationand has for its principal object producing efiicient, high fidelityamplification of electrical voltage and power over a wide range offrequencies.

A further object is to provide a high fidelity amplifier with a highdegree of reliability and capacity to deliver high overloads atfrequencies down to substantiallyzero frequency or direct current aswell as in the video region of the frequency spectrum.

A further object of the invention is to obtain prompt and faithfulresponse to transient conditions and high fidelity amplification ofspeech and other signals of constantly varying frequency and amplitudewhich make the transient characteristics of the amplifier of as greatimportance as the characteristics under steady state conditions. Anotherobject of the invention is to make'the magnitude and phase of the outputvoltage substantially independent of load impedance variations thatwould normally be encountered and to provide a generator having thepower to be the principal factor in the determination of magnitude andphase of the output voltage.

Still another object is to apply corrective feedback for the reductionof distortion Without creating undesirable transients. I

A further object is to eliminate the necessity for directcurrent flow inthe load circuit, transformer coils, choke coils or devices which may besubject to magnetizing or to other unilateral effects.

An additional object is to permit grid excitation toenter the positivegrid region without creating distortions 'or transients in the couplingcircuits. 7 V

A further object is to accomplish maximum coupling elficiency andminmium reactive eifects in a simple circult.

Still another object is to permit the free interchange of direct currentand direct-current potentials. It is also an object to make correctivefeedback fully effective at direct current and to obtain greatstability.

Other and further objects, features and advantages of the invention willbecome apparent as the description proceeds.

1 In carrying out the invention in accordance with a preferred formthereof, I provide a pair of direct coupled, output tubes, their gridsdriven in opposition, supplying a single ended output in parallel,thereby eliminating a requirement of supplying a substantial unitymagnetic coupling between the output circuits of two opposing outputtubes. The grids of the output tubes are supplied in opposite phase by aphase splitter circuit. Preferably, an input amplifier tube is provided,having a direct feedback connection from the output circuit. For thesakeof Obtaining a most faithful wave reproduction and permitting highpositive peak currents to be supplied at high peak voltage, cathodefollower connections are provided for exciting the grids of both theoutput tubes.

A better understanding of the invention will be afforded by thefollowing detailed description considered in conjunction with theaccompanying drawings, in which Fig. 1 is a circuit diagram of apreferred embodiment of the invention; and

'Fig. 2 is a simplified circuit diagram illustrating a manner ofcarrying out the invention with the use of triode vacuum tubes.

Fig. 3 is a diagram of a further simplified circuit.

Like reference characters are utilized throughout the drawings todesignate like parts.

In the arrangement illustrated in Fig."1, a pair of electronic dischargetubes T1 and T2 of the grid controlled type are providing having asuitable plate supply. By way of example, the plate power supply isillustrated as taking the form of a step-up transformer 11 energizing avoltage doubling rectifier circuit consisting of a pair of rectifierunits 12 and 13, connected across a pair of condensers 14 and 15,connected in series between the plate input terminal lines representedas a grounded line 16 and positive high potential line 17. The secondarywinding of the transformer 11 is connected to the common terminal 13 ofthe condensers 14 and 15 and to the common terminal 19 of the rectifierunits 12 and 13. This circuitis preferred because it automaticallyprovides a fixed appropriate screen potential for tubes T2 and T3.

As shown, the tube T1 has an anode 21 connected to the positive side 17of the plate supply and the tube T2 has a cathode 22 connected to thenegative side 16 of the plate supply. The tube Ti has a cathode 23connected to, the anode 24 of the tube T2. If the tubes T1 and T2 takethe form of tetrode vacuum tubes, as illustrated, screen potential isprovided by a dropping resistance 25 connected between the positiveplate supply 17 and the screen grid 26, and a screen-voltage condenser27 is connected between the screen grid 26 and the cathode 23. Thescreen potential for the tube T2 is provided, by a dropping resistance28 connected between the mid-terminal 18 of the plate power supply andthe screen grid 29 of the tube T2. In this case the condenser 15 servesalso for stabilizing screen voltage. The output from the circuit istaken from the common terminal 31 of the tubes T1 and T2. For example,as illustrated, there may be a grounded terminal 32 and an ungroundedoutput terminal 33 coupled to the parallel output terminal 31 from thetubes T1 and T2 through a condenser 30.

V For exciting grids or control electrodes 34 and 35 of the tubes T1 andT2 in phase opposition, a phase splitter is provided which may take theform of an electronic discharge tube T shown as a pentode vacuum tube inFig. 1, having an anode 36 connected to the positive side 17 of theplate supply through an anode resistor 37 and having a cathode 38connected through a cathode resistor 3 preferably to a point belowground potential, such as a terminal 41 of a negative voltage powersupply 42. The anode 36 of the tube T3 is coupled to the grid 34 of thetube Ti and the cathode 38 of the tube T3 is coupled to the grid 35 ofthe tube T2. For the latter coupling, a direct connection is providedsince the tube T3 acts as a cathode follower.

If large quantities of positive-current peak power are to be handled,preferably a cathode follower T4 is inserted between the anode 36 of thetube T and the grid 34 of the tube T1. As illustrated, the tube T4 maytake the form of a triode vacuum tube having an anode 43 connected tothe positive side 17 of the plate power supply and a cathode 44connected through a cathode resist-or 45 to the negative or groundedside 16 of the plate power supply, the cathode 44 being directlyconnected to the grid 34 of the tube T1 The tube T3 may have a screengrid 46, voltage for which is provided by a suitable source, forexample, as

Patented Jan. 8, Th5? shown, the mid-terminal 18 of the power supply, towhich the screen grid of the tube T3 (equivalent to a triode anode) isdirectly connected in order to supply full power to the cathode 28 ofthe tube as a cathode follower. The input signal is supplied to thephase splitter tube T3 through a control electrode or grid 47.

Preferably, an input amplifier tube T is provided shown as a triodevacuum tube having an anode 48 connected through an anode resistor 49 tothe positive side 17 of the plate power supply, a cathode 51 connectedthrough a cathode resistor 52 to a suitable point such as a terminal 53in the negative power supply 42 and a control electrode or grid 54 towhich the input signal terminal 55 is connected. The anode 48 is coupledto the grid 47 of the phase splitter tube T 3, preferably by a directconnectron where maximum fidelity is desired. For obtaining correctivefeedback, effective at all frequencies, even at direct current, a directconnection is made between the cathode 51 of the input amplifier tube T5and the output terminal 31 of the output tubes T1 andTa, by connecting aconductor 56 between the cathode 51 of the tube T5 and theanode 24 ofthe tube T2, preferably with a current hmiting resistor 57 interposed inthe connection.

In order to maintain a suitable negative bias on the grid 54 of theinput amplifier tube T5, a voltage divider is provided, consisting ofresistors R5 and Re, which are connected in series between the groundedsignal terminal 60 and the negative terminal 53, which is the negativeterminal forthe circuit of the tube T5. The input term1nal55 to whichthe grid 54 is connected, is as shown, the common terminal or junctionterminal of the voltage divider terminals R5 and Re.

The negative power supply 42 may be of conventional form consisting asillustrated, of a rectifier unit 58 supplied from another secondarywinding of the transformer 11, including conventional filter condensers59 and series resistors 61 with a voltage regulator gas type tube 62 forthe negative power supply terminal 41, and a filter circuit consistingof a resistor 63 and a condenser 64 for the negative supply terminal 53.It will be understood, however, that a separate negative power supply isnot required and suitable relative potentials or biases of the variouselectrodes may be provided in any desired manner known to those skilledin the art.

The two output tubes T1 and T2 are direct coupled 111 order to obtainthe unity coupling that is necessary between these tubes to preventcreation of transients which are otherwise unavoidable in push-pullclass B circuits in which the output tubes are alternately driven beyondplate current cutoff. The direct coupling between Tiand T2 connectsthese two tubes in parallel, and since T1 is a cathode follower, thereis imparted to this output circuit an extremely low output impedance orgenerator impedance, which gives this generator the power to be theprincipal factor in the determination of the magnitude and phase of theoutput voltage, thereby maintaining these quantities substantiallyindependent of load impedance variations that would normally beencountered. Under these conditions, since the generator impedanceremains a substantially pure resistance over the entire range of normaloperation, corrective feedback for the reduction of distortion can beapplied without creating the undesirable transients which must occur inany amplifier with degenerative feedback in which the load impedancevaries and can affect the phase of the output voltage from which thefeedback voltage is derived. This connection of T1 and T2 furthermoreeliminates the necessity of direct current flow in the load circuit, thetransformer coils, or other choke coils because the tubes mutuallysupply initial plate current to each other; and, during operation, thesetubes alternately valve only signal currents into the load.

, Tube T3 is a direct coupled driver for T1 and T2. This arrangementpermits grid excitation to enter the positive grid region withoutcreating distortions or transients in the coupling circuits. Maximumsimplicity, maximum coupling efiiciency, and minimum reactive effectsarealso achieved by this driver circuit.

The cathodel follower T4 is connected between T3 and T1 to provide theextra grid current required by T1 during high power output peaks. Thecathode follower has the fortunate property of being especially able tosupply high positive peak currents at high voltage. This is exactly therequirement for efficiently exciting T1. T4 furthermore, draws verylittle current from the power supply except when high current is neededto drive T1 during which interval T4 actually delivers some of itsoutput power directly to the amplifier load through the electron circuitwhich exists during this interval between the cathode and grid of T1.This extra power is just what is needed to help reduce distortion duringhigh power peaks.

The voltage amplifier T5 provides the final step for the creation of ahigh fidelity amplifier that is capable of reproducing wave forms withgreat accuracy. The amplifier stage usingTs is designed for maximumgain, minimum phase shift, and maximum cathode to ground irn pedance inorder that the maximum amount of corrective feedback may be applied withgreatest simplicity, greatest effectiveness, and at the expense of aminimum amount of the output power.

All circuitsin this amplifier are direct coupled to permit the freeinterchange of direct current and direct current potentials. Thisapplies also to the inverse or negative feedback circuit. Since thecorrective feedback is thereby made fully effective at direct current,the amplifier possesses an extreme degree of stability. It is highlyinsensitive to variations in tube characteristics and resistor valuesexcept that the ratio of Rs/Rs must be accurately maintained for maximumpower output capability. R5 and R6 do not benefit from the selfstabilizing property of the amplifier because these resistors mustnecessarily be outside of the feedback loop. Operation of the outputtubes of the amplifier is not limited to the negative grid regionbecause direct coupling permits the transfer of power to the gridswithout setting up serious transients. This is further exploited byproviding low impedance circuits to drive these grids, therebyincreasing the amplifier efiiciency, increasing the available power fromany given tube and reducing the load resistance required for optimumtermination of the amplifier.

As illustrated, in the case of the audio version of the,

amplifier, as well as some other possible versions, plate supply is fedto the output tubes connected in series and only minor currents canenter the point in this circuit between the outputtubes, the averageplate current flowing in these two tubes, must therefore, remainsubstantially equal and a balanced condition of operation iscontinuously maintained.

When output tubes such as the 6V6 or 6L6 are used in the amplifier theoptimum termination resistance for maximum power output for a pair ofthese tubes is in the order of 1000 ohms.

The output from the amplifier, in the absence of an output transformer,normally appears on two terminals, one of which is hard at groundpotential. Only signal currents flow between these terminals as would bethe case if an output transformer were used. It follows therefore, thatit is practical to build the amplifier without providing space for anoutput transformer on the assumption that there is much more inexpensivespace on the reproducer unit or in its cabinet for mounting the outputtransformer. In multiple reproducer sound reinforcing systems, thedistribution line can readily be fed without a transformer at theamplifier.

The effectiveness of the corrective feedback at direct currentspractically eliminates any necessity for selecting tubes to obtainoptimum or maximum performance. Through the action of the direct currentfeedback, the tube biases are automatically adjusted for very nearlyoptimum performance of the amplifier in the presence of wide variationsof the tube characteristics extending in some cases even to the use oftubes of different type numbers. This is especiallytme in the case ofthe output tubes.

My invention is not limited to the use of particular tube types nor toexact electrical dimensions of circuit components. Nevertheless, I havefound that good results over wide range of frequencies may be obtainedwhere the tubes T1 and T2 are 6V6 tubes supplied at 500 volts, with thetransformer 11, stepping up from 117 volts to 260 volts A. C.,condensers 14 and 15 having capacity of 80 microfarads,condenser 27,capacity of 20 microfarads, the condenser 30 having a capacity of 50microfarads, resistor 25 having 8000 ohms; resistor 28, 2000 ohms;cathode resistor 45, 250,000 ohms, the anode resistor 37, A megohm,cathode resistor 39, 5000 ohms; cathode resistor 52, 10,000 ohms, R andRs, 1 megohm and 3 megohms, respectively; anode resistor 49, 2 megohms;terminal 41 supplying minus 55 volts and terminal 53 supplying minus 145volts.

The particular tube types to be employed, will of course, be variedaccording to the power output required. Moreover, the circuit is notcritical as to the selection of tubes and it is not necessary to matchtubes or to select from tubes of the same type in order to obtain a tubewhich will have precise characteristics in order to obtain satisfactoryoperation of the circuit. As illustrated in Fig. 2, tubes T1 and T2 andT3 may be replaced by triode vacuum tubes and if the positive-currentpeak power requirements are not excessive, the cathode follower tube T4may be eliminated with a direct connection made between the anode 48 ofT5 and the control electrode 47 of the tube T3.

While I have described my invention as embodied in concrete form and asoperating in a specific manner in accordance with the provisions of thepatent statutes, it should be understood that I do not limit myinvention thereto, since various modifications thereof will suggestthemselves to those skilled in the art without departing from the spiritof my invention.

What is claimed is:

1. A direct coupled high-fidelity wide-range amplifier comprising incombination a pair of first and second vacuum tubes, each having ananode, a cathode and a control electrode, the anode of the second vacuumtube being connected to the cathode of the first to serve as an outputterminal, and the remaining anode and cathode being adapted to beconnected to a power supply, a load coupling circuit comprising acapacitor connected to the cathode of the first vacuum tube and adaptedto have a load connected thereto and to the cathode of the second vacuumtube, a phase-splitter tube having a control electrode, an anodeconnected to the control electrode of said first vacuum tube and acathode connected to'the control electrode of the said second vacuumtube, and having anode and cathode resistors, an input tube having acontrol electrode serving as the input terminal for the amplifier, ananode connected to the control electrode of the phase splitter tube, acathode connected to the said cathode of the said first vacuum tube, andanode and cathode resistance in series with said input tube, and acathode follower tube interposed in the connection between the anode ofsaid phase splitter tube and the control electrode of said first vacuumtube, said cathode follower tube having a control electrode connected tothe anode of the phase splitter and a cathode connected to the controlelectrode of the said first vacuum tube.

2. A direct coupled high-fidelity wide-range amplifier for energizationfrom a powersupply, having positive and negative terminals, saidamplifier comprising a pair of first and "second vacuum tubes,eachhaving an anode, a cathode and a control electrode, the anode of thesec-- ond vacuum tube being connected to the cathode of the first 'toserve as an output terminal, and the remaining anode and cathode beingadapted to be connected to the power supply terminals, phase-splittertube means having control electrode means, an anode connected to thecontrol electrode of said first vacuum tube and a cathode connected tothe control electrode of the said second vacuum tube, and having anodeandcathode resistors, an input tube having a control electrode servingas the input terminal for the amplifier, an anode connected to thecontrol electrode means of the phase splitter tube means, a cathodeconnected to the said cathode of the said first vacuum tube, and anodeand cathode resistance in series with said input tube, and a cathodefollower tube interposed in the connection between the anode of saidphase splitter tube and the control electrode of said first vacuum tube,said cathode follower tube having a control electrode connected to thesaid anode of the phase splitter means and a cathode connected to thecontrol electrode of the said first vacuum tube.

3. A direct coupled wide-range amplifier for energization from a powersupply with a plurality of fixed potential terminals including positiveand negative terminals, said amplifier comprising a pair of first andsecond vacuurn tubes, each having an anode, a cathode and a controlelectrode, the anode of the second vacuum tube being connected to thecathode of the first to serve as an output terminal, and the remaininganode and cathode being adapted to be connected to the power supplyterminals, a phase-splitter tube having a control electrode, a screengrid, an anode connected to the control electrode of said first vacuumtube and a cathode connected to the control electrode of the said secondvacuum tube, said screen grid being connected to a fixed potentialterminal in the power supply and having anode and cathode resistances, acathode follower tube interposed in the connection between the anode ofthe phase splitter tube and the control electrode of the first vacuumtube and an input tube having a control electrode serving as the inputterminal for the amplifier, an anode connected to the control electrodeof the phase splitter tube, a cathode conductively connected to the saidcathode of the said first vacuum tube, and anode .and cathode resistancein series with said input tube,

said cathode follower tube having a control electrode connected to thesaid anode of the phase splitter tube and a cathode connected to thecontrol electrode of the said first vacuum tube.

4. A vacuum-tube amplifier for energization from a power supply,comprising a pair of first and second vacuum tubes, each having ananode, a cathode and a control electrode, the anode of the second vacuumtube being connected to the cathode of the first to serveas an outputterminal, and the remaining anode and cathode being adapted to beconnected to the power supply terminals, phase-splitter tube meanshaving control electrode means, an anode, additional electrode meansadapted to be connected to a power supply terminal, and a cathodeconnected to the control electrode of the said second vacuum tube, andhaving anode and cathode loading, a cathode follower tube having ananode adapted to be connected to a power supply terminal, a controlelectrode connected to the anode of said phase splitter tube, and acathode connected to the control electrode of said first vacuum tube,and an input tube having acontrol electrode serving as the inputterminal for the amplifier, an anode connected to the control electrodeof the phase splitter tube, a cathode conductively connected to the saidcathode of the said first vacuum tube, and anode and cathode loading.

. 5. A vacuum tube amplifier comprising a cathode follower vacuum tubehaving a cathode, a second vacuum tube having an anode connected tothecathode of said cathode follower vacuum tube, an input tube having acathode .and being conductively coupled to the second vacuum tube and tothe cathode follower tube and feedback conductive connection from thejunction of the second vacuum tube and the cathode-follower tube to saidinput tube cathode.

References Cited in the file of this potent" UNITED. STATES PATENTS2,310,342 Artzt Feb; 9, 1943 10 2,431,973 White Dec. 2, 1947 2,438,960

Blitz Apr. 6, 1948 '8 2,488,567 Stodola -.Nov. 22, 1949: 2,549,833Martinez "Q. Apr. 24, 1951 2,631,198 Parisoe Mar. 10, 1953

